1. Field of the Invention
This invention relates to a method and apparatus for combining resin bonding and mechanical anchoring of a roof bolt in a bore hole of a rock formation and more particularly to a mine roof support system that includes the features of both a point anchor resin roof bolt and a mechanical anchor roof bolt.
2. Description of the Prior Art
It is well known to reinforce and stabilize underground ock formations, such as a coal mine roof, a subway tunnel or similar subterranean strcutre or to strengthen a rock mass by the use of anchor bolts inserted within a bore hole drilled in the rock formation. The anchor bolts are tensioned during installation to reinforce the unsupported rock formation above the roof, for example, above the roof of a mine passageway. Conventionally, a hole is drilled through the roof into the rock formation. The end of the bolt in the rock formation is anchored either by engagement of a mechanical expansion shell with the wallof the rock formation around the bore hole or chemically anchoring the bolt by a multicomponent resin system or grout to the rock formation surrounding the bore hole. With a chemical anchor, after the resin or grout is mixed by rotation of the bolt, the material penetrates into the surrounding rock formation to adhesively unite the rock strata and anchor, by bonding, the bolt to the rock formation surrounding the bore hole. The mixed resin or grout fills the annulus between the bore hole wall and the bolt along a substantial length of the bolt.
U.S. Pat. Nos. 3,108,443; 3,892,101; 3,940,941; 3,974,918; 4,051,683; 4,127,000; 4,129,007; 4,263,832; and 4,303,354 are examples of systems that use a grout or resin to anchor a roof bolt in a rock formation. U.S. Pat. Nos. 3,925,996 and 4,216,180 disclose a multicomponent system for chemically anchoring a roof bolt in a rock formation. These resin systems are quick-setting whereby, once the components are mixed, the mixture cures and begins to harden within seconds after mixing.
In a multicomponent resin system, the components are separated until the time that mixing and setting is desired. The components are maintained separated in compartments of a fragmentable cartridge. The cartridge is inserted into the bore hole in the roof and the roof bolt is pushed into the bore hole to advance the cartridge to the closed end of the bore hole where the cartridge is ruptured and the chemical components are mixed upon continued rotation of the bolt.
It has been the conventional practice to rotate the bolt for a period of time to thoroughly mix the components in order to insure a complete cure of the chemical mixture. Thereafter, once the chemical mixture cures and hardens, the bolt is bonded to the wall of the rock formation in the bore hole. It is disclosed in U.S. Pat. No. 4,216,180 that once the resin cartridge is ruptured, components are mixed and the mixture cures without requiring rotation of the bolt.
A roof bolt which is mechanically anchored in a bore hole, is placed in tension once the upper end of the bolt is anchored in the bore hole by compression of a roof plate into contact with the face of the rock formation surrounding the opening into the bore hole. Tensioning an anchored mine roof bolt compresses the rock strata to reinforce the strata to resist shifting of the strata above a mine passage or an underground tunnel.
U.S. Pat. Nos. 3,940,941; 3,979,918; 4,303,354; and 4,386,877 disclose point anchor resin roof bolt support systems in which the roof bolt is chemically bonded to the rock formation and is placed in tension after the mixed resin is allowed to harden. A multicomponent resin cartridge system or grout is advanced by the upper end of the roof bolt into the bore hole. The bolt and the cartridge system are advanced into the bore hole to a position where the threaded end of the bolt emerges from the bore hole. A roof plate positioned on the emerging end of the bolt abuts against the face of the rock formation around the bore hole. A nut is placed on the threaded end of the bolt below the roof plate.
The bolt is rotated to effect mixing of the components of the resin system or grout. After a short period of time, for example, 5 seconds in the case of a quick-setting resin, rotation is stopped to permit curing of the mixed resin components for a preselected period of time, for example, 30 to 60 seconds. After the resin is cured and has hardened sufficiently, the nut is rotated on the emerging end of the bolt but the bolt does not rotate due to the resistance to rotation presented by the cured resin surrounding the bolt. The nut is tightened against the roof plate to put the roof plate under the desired tension.
In one method of operation, the roof bolt and the nut are rotated in one direction to effect mixing of the resin components. After the components are mixed and allowed to harden, during which time the bolt is not rotated, the nut is rotated in an opposite direction on the bolt to place the bolt in tension. In another method of operation, the direction of rotation for mixing the resin components is in the same direction of rotation of the nut on the bolt to tension the bolt. In both of these cases, the nut is provided with a stop or shearable means for resisting relative rotation between the nut and the bolt until the cured resin offers sufficient resistance to rotation of the bolt and continued rotation of the nut advances the nut on the bolt.
U.S. Pat. Nos. 3,877,235; 4,023,373; 4,051,683; and 4,275,975 disclose a chemically anchored roof bolt system that includes an anchor portion which is inserted into the bore hole behind a resin cartridge and a lower portion connected to the anchor portion. In U.S. Pat. No. 4,023,373, the anchor portion is a pipe-like, hollow, cylindrical member into which extends the threaded end of a bolt. In U.S. Pat. No. 4,051,683, the anchor member is a rebar connected by a coupling to a bolt which supports the roof plate. With these devices, once the resin components have been mixed and the mixture cured and sufficiently hardened to adhesively secure the anchor portion in the bore hole, application of a predetermined amount of torque to the bolt below the anchor releases the bolt for rotation relative to the anchor to draw the roof plate into compressive relation withthe mine roof and thereby put the bolt under tension. The anchor portion is not tensioned, only the bolt is tensioned. These systems do not utilize an expansion shell to anchor the roof bolt in the bore hole.
U.S. Pat. Nos. 4,413,930 and 4,419,805 disclose method and apparatus for combining resin bonding and mechanical anchoring of a bolt in a rock formation. With these devices, a single bolt with a mechanical anchor threaded onto the upper end of the bolt is inserted into the bore hole behind the resin cartridge. A roof plate is carried on the opposite end of the bolt for abutment against the rock formation surrounding the open end of the bore hole. The cartridge is ruptured by the upward thrust and rotation of the bolt to release the resin components for mixing. A stop device associated with the expansion shell restrains expansion of the shell during rotation of the bolt in a preselected direction to mix the resin components. Rotation of the bolt continues without expansion of the shell for a predetermined period of time to permit the mixed resin to cure. As the resin mixture begins to harden and the bolt encounters resistance to rotation, the stop device is released to permit expansion of the shell into engagement with the wall of the bore hole before the mixed resin completely hardens. With this arrangement, the bolt is continuously rotated from the initial insertion to the point where the preselected torque has been applied to the bolt to put the bolt under the desired tension. thus,the bolt is both mechanically and chemically anchored in the bore hole.
While it has been suggested by the prior art point anchor resin roof bolt devices to chemically anchor a roof bolt in a rock formation and also put the bolt under tension, the prior art devices require complete mixing and curing of the resing before the bolt is finally installed under tension. This required that the bolt be initially rotated to mix the resin components and rotation interrupted for a period of time to permit the resin mixture to cure. Thereafter, the bolt or a nut on the bolt is rotated in a preselected direction to put the bolt under a desired degree of tension. Even though the development of quick-setting resins has shortened the hold time before the bolt can be fully tensioned, interruption of the bolt rotation is still required. Therefore, there is need, in the chemical anchoring of a roof bolt in rock formation, for a system that permits continuous uninterrupted rotation of the bolt once it is inserted in the bore hole to the point of final tensioning where the resin components or grout are mixed and cured without interrupting the initial rotation of the bolt to put the bolt under tension.